1. Field of the Invention
The present invention relates to the field of grid-type axial-beam electron tubes and especially to those with inductive output, known by the abbreviation IOT (standing for Inductive Output Tube). It relates more particularly to the grid of these tubes.
2. Discussion of the Background
An IOT comprises an electron gun which emits an electron beam directed along a longitudinal axis, this beam passing through a resonant cavity with which it interacts, before being received in a collector which adjoins the resonant cavity.
In these tubes, the gun has a cathode generally with a concave emissive part, a heater, a control grid and an anode, the grid being located between the cathode and the anode.
The grid serves to modulate the emission of electrons so that they are grouped in packets as soon as they are emitted by the cathode. The beam thus modulated passes through the single cavity from which electromagnetic energy is extracted. These tubes have an efficiency and a high gain.
FIG. 1 illustrates highly schematically a known electron gun for an IOT-type tube. The thermoemissive cathode bears the reference 1. It is solid, with a concave emissive face 2. A heater 3, which heats by conduction or radiation, is located opposite the emissive surface of the cathode 1. The control grid bears the reference 4. It is placed so as to face the emissive surface 2 of the cathode 1. It is very close to the latterxe2x80x94the gap which separates them may be of the order of a few tenths of a millimetre.
Next, there is an anode 5 provided with a central opening 6.
The electrons form a beam (not shown) directed along the XXxe2x80x2 axis. In this beam, the electrons are grouped in packets as soon as they pass through the central aperture 6. Beyond the central opening 6, they penetrate into the body of the tube (shown in dotted lines), from the resonant cavity as far as the collector.
The grid 4 has an apertured part 7 with bars in a central region and a peripheral solid part 8, the assembly being in the form of a disc which is substantially plane or slightly concave in order to follow the emissive surface 2 of the cathode 1. The grid 4 is delicate and its bars are thin. The grid 4 is intended to be connected to a grid connection piece 10 located at the base of the gun, on the opposite side from the grid 4 with respect to the heater 3. It receives via this piece 10 an electrical modulation signal. This connection is made by means of an electrically conducting support 9 fastened on one side to the solid part 8 of the grid 4 and on the other side to the grid connection piece 10. This support 9 is formed by joining together several approximately cylindrical pieces, one of which, 91, surrounds the grid 4.
Also around the base of the gun, in addition to the grid connection piece 10, there is a cathode connection piece, a heater connection piece and an anode connection piece. These connection pieces are not shown. They are separated from one another by dielectric spacers. These connection pieces 10 are remote from the cathode 1 and from the heater 3, and are therefore not exposed to high temperatures.
The support 9 for the grid 4 is located close to the cathode and it surrounds it. It is generally made of metal because of its electrical properties, since it contributes to transmitting the modulation signal to the grid 4.
In the prior art, and for thermoelectric property reasons, the grid 4 is made of pyrolytic graphite, a material known for its very small expansion coefficient in the deposition plane.
The grid 4 is subjected to high thermal and electrical stresses, but in order to fulfil its role and to modulate the beam effectively, it must be capable of withstanding them without deforming.
The grid-cathode gap must remain approximately constant during operation of the tube this is an important parameter in the effectiveness of the modulation and the stability of the signal.
The grid 4 heats up, on the one hand because of its closeness to the thermoemissive cathode 1 and on the other hand because of the emitted electrons which inevitably strike it.
When the tube is in use, a differential expansion between the grid 4 and its support 9 occurs since they do not have the same expansion coefficient. The grid is generally made of pyrolytic graphite and the support 9 made of metal.
This differential expansion results in stresses on the grid 4 which may deform the apertured region 7 if the grid is tightly fastened to the support 9 and may cause the grid 4 to come into contact with the cathode 1 or, at the very least, may alter the modulation of the electron beam.
It has been proposed, as in FIG. 1, to mount the grid 4 on its support 9 in an elastic manner with the aid of an elastic joint 11 which is electrically conductive. This joint absorbs the differences in expansion. A relative sliding movement between the grid 4 and the support 9 is possible during the expansion, thereby preventing excessively high mechanical stresses in the grid 4. However, it is difficult to ensure that the cathode and the grid are coaxial. There is a risk of the gap between the grid and the cathode varying. Another drawback with this structure is that it is expensive to produce. The elastic joint 11 must leave a mechanical clearance between the grid 4 and the support 9 without correspondingly interrupting the electrical continuity between the two. Fitting the joint is tricky and requires a complicated support 9 with several pieces joined together.
The present invention seeks to alleviate these drawbacks and to this end provides a grid for an axial-beam electron tube with improved performance which is particular simple to produce. It results in an inexpensive gun which makes it possible, in operation, to maintain an accurately defined cathode-grid distance independent of the temperature stabilization time of the various electrodes. It makes it possible to dispense with the metal grid support.
More particularly, in order to achieve this, the grid according to the invention has the shape of a bell and is made of a single material, this bell having at its top an apertured part, this part being approximately transverse to the axis of the beam.
The grid will preferably be made of pyrolytic graphite because of its thermal, electrical and mechanical properties which are suitable for this type of application. Preferably, the grid is made as one piece.
To improve the focusing of the electrons which pass through the grid around the periphery of the apertured part, it is possible for the top of the bell to be configured in the form of a hollow, to place the apertured part in the bottom of the hollow and to border the apertured part with a tubular wall attached to the skirt by an annular part which separates it from the skirt.
The grid is intended to be fastened at the base of the bell to a grid connection piece.
This grid connection piece may comprise a sleeve around which the grid is fitted.
The fastening between the grid and the grid connection piece may be achieved, for example, by soldering or screwing. In order to further improve the performance of the grid, it is advantageous to provide a series of slots oriented approximately along the axis of the electron beam at the base of the grid. This series of slot allowing elastic compensation of the differences in expansion that can occur between the grid and the grid connection piece.
The present invention also relates to an axial-beam electron tube equipped with such a grid.